Under federal regulation, you can and are allowed to make a homemade gun. you have to follow the regulations set for pistols, shotguns, and rifles. If you are wanting to build an NFA item, you can request permisson from ATF to do such. Once it is built, you will have to go through the tax process like any other weapon.
However, to note, once you have built a homemade firearm, you can not transfer and/or sell it. That would be a violation of tax code and you will be subject to criminal prosecutetion.
You can find the information you are looking for on BATF's website.
I never saw a wild thing sorry for itself. A small bird will drop frozen dead from a bough. Without ever having felt sorry for itself. -- D.H. Lawrence